The present invention relates to a device for detecting engine knock for suitably controlling the operating condition of an internal combustion engine by sensing ringing vibrations produced in the engine as engine knock.
Such a device has been proposed in U.S. Pat. No. 3,822,583, No. 4,002,155 and No. 4,012,942. In the conventional internal combustion engine, the ignition timing closely related to engine knock is noted, the condition of engine knock is sensed, the spark or ignition timing is adjusted correspondingly, a permissible condition of engine knock is maintained, and fuel costs and output performance of the engine are improved. An engine knock sensing device adopting such a device is, for example, shown in FIG. 1.
FIG. 1 shows a block diagram and simple signal waveform of the engine knock sensing device. In FIG. 1, reference numeral 1 is an engine knock sensor secured to an engine body for sensing vibrations of the engine. The sensor 1 generates an AC voltage in accordance with vibrations due to engine operation, but its resonance frequency is matched with a ringing vibration frequency due to engine knock, so that the voltage generated by the ringing vibrations due to engine knock is particularly large. The output voltage of the sensor 1 is rectified by a half-wave rectifier 2, and divided and amplified in an amplifier 3, one output of which is directly supplied to a comparator circuit 4 and the other output of which is supplied to the comparator circuit 4 through a smoothing circuit 5.
The smoothing circuit 5 generates a mean value of the amplified and rectified waveform and amplifies the mean value to a predetermined multiple. The comparator circuit 4 counts the number of a peak values of the voltage supplied from the amplifier 3 exceeding the mean value voltage by every certain period, for example, each ignition interval and senses that the condition of the engine is ringing or knocking when the number becomes more than a certain number. Reference numeral 6 is an ignition timing control circuit for controlling the spark or ignition timing by an output signal of the comparator circuit 4.
In such a prior device, however, the output of the engine knock sensor is considerably different in magnitude according to the case whether the engine knock is produced or not, so that the output of the smoothing circuit 5 is considerably varied accordingly, and thus, a mean value of output of the circuit 5 at the time of engine knock becomes larger than the other time. Therefore, in the comparator circuit 4, a relative difference between the peak value and the mean value of the sensor output of the time of engine knock becomes small, so that it is difficult to compare both the values with each other, and a sensitivity to engine knock becomes lowered.